IMA: add support to restrict the hash algorithms used for file appraisal

The kernel accepts any hash algorithm as a value for the security.ima
xattr. Users may wish to restrict the accepted algorithms to only
support strong cryptographic ones.

Provide the plumbing to restrict the permitted set of hash algorithms
used for verifying file hashes and signatures stored in security.ima
xattr.

Signed-off-by: THOBY Simon <Simon.THOBY@viveris.fr>
Reviewed-by: Lakshmi Ramasubramanian <nramas@linux.microsoft.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

Author: nightmared

security/integrity/ima/ima.h

@@ -47,7 +47,7 @@ enum tpm_pcrs { TPM_PCR0 = 0, TPM_PCR8 = 8, TPM_PCR10 = 10 };
 extern int ima_policy_flag;
 
 /* set during initialization */
-extern int ima_hash_algo;
+extern int ima_hash_algo __ro_after_init;
 extern int ima_sha1_idx __ro_after_init;
 extern int ima_hash_algo_idx __ro_after_init;
 extern int ima_extra_slots __ro_after_init;
@@ -254,7 +254,7 @@ int ima_get_action(struct user_namespace *mnt_userns, struct inode *inode,
 		   const struct cred *cred, u32 secid, int mask,
 		   enum ima_hooks func, int *pcr,
 		   struct ima_template_desc **template_desc,
-		   const char *func_data);
+		   const char *func_data, unsigned int *allowed_algos);
 int ima_must_measure(struct inode *inode, int mask, enum ima_hooks func);
 int ima_collect_measurement(struct integrity_iint_cache *iint,
 			    struct file *file, void *buf, loff_t size,
@@ -285,7 +285,7 @@ int ima_match_policy(struct user_namespace *mnt_userns, struct inode *inode,
 		     const struct cred *cred, u32 secid, enum ima_hooks func,
 		     int mask, int flags, int *pcr,
 		     struct ima_template_desc **template_desc,
-		     const char *func_data);
+		     const char *func_data, unsigned int *allowed_algos);
 void ima_init_policy(void);
 void ima_update_policy(void);
 void ima_update_policy_flag(void);

security/integrity/ima/ima_api.c

@@ -172,6 +172,7 @@ void ima_add_violation(struct file *file, const unsigned char *filename,
  * @pcr: pointer filled in if matched measure policy sets pcr=
  * @template_desc: pointer filled in if matched measure policy sets template=
  * @func_data: func specific data, may be NULL
+ * @allowed_algos: allowlist of hash algorithms for the IMA xattr
  *
  * The policy is defined in terms of keypairs:
  *		subj=, obj=, type=, func=, mask=, fsmagic=
@@ -188,14 +189,15 @@ int ima_get_action(struct user_namespace *mnt_userns, struct inode *inode,
 		   const struct cred *cred, u32 secid, int mask,
 		   enum ima_hooks func, int *pcr,
 		   struct ima_template_desc **template_desc,
-		   const char *func_data)
+		   const char *func_data, unsigned int *allowed_algos)
 {
 	int flags = IMA_MEASURE | IMA_AUDIT | IMA_APPRAISE | IMA_HASH;
 
 	flags &= ima_policy_flag;
 
 	return ima_match_policy(mnt_userns, inode, cred, secid, func, mask,
-				flags, pcr, template_desc, func_data);
+				flags, pcr, template_desc, func_data,
+				allowed_algos);
 }
 
 /*

security/integrity/ima/ima_appraise.c

@@ -77,8 +77,9 @@ int ima_must_appraise(struct user_namespace *mnt_userns, struct inode *inode,
 		return 0;
 
 	security_task_getsecid_subj(current, &secid);
-	return ima_match_policy(mnt_userns, inode, current_cred(), secid, func,
-				mask, IMA_APPRAISE | IMA_HASH, NULL, NULL, NULL);
+	return ima_match_policy(mnt_userns, inode, current_cred(), secid,
+				func, mask, IMA_APPRAISE | IMA_HASH, NULL,
+				NULL, NULL, NULL);
 }
 
 static int ima_fix_xattr(struct dentry *dentry,

security/integrity/ima/ima_main.c

@@ -215,6 +215,7 @@ static int process_measurement(struct file *file, const struct cred *cred,
 	int xattr_len = 0;
 	bool violation_check;
 	enum hash_algo hash_algo;
+	unsigned int allowed_algos = 0;
 
 	if (!ima_policy_flag || !S_ISREG(inode->i_mode))
 		return 0;
@@ -224,7 +225,8 @@ static int process_measurement(struct file *file, const struct cred *cred,
 	 * Included is the appraise submask.
 	 */
 	action = ima_get_action(file_mnt_user_ns(file), inode, cred, secid,
-				mask, func, &pcr, &template_desc, NULL);
+				mask, func, &pcr, &template_desc, NULL,
+				&allowed_algos);
 	violation_check = ((func == FILE_CHECK || func == MMAP_CHECK) &&
 			   (ima_policy_flag & IMA_MEASURE));
 	if (!action && !violation_check)
@@ -361,6 +363,16 @@ static int process_measurement(struct file *file, const struct cred *cred,
 
 	if ((file->f_flags & O_DIRECT) && (iint->flags & IMA_PERMIT_DIRECTIO))
 		rc = 0;
+
+	/* Ensure the digest was generated using an allowed algorithm */
+	if (rc == 0 && must_appraise && allowed_algos != 0 &&
+	    (allowed_algos & (1U << hash_algo)) == 0) {
+		rc = -EACCES;
+
+		integrity_audit_msg(AUDIT_INTEGRITY_DATA, file_inode(file),
+				    pathname, "collect_data",
+				    "denied-hash-algorithm", rc, 0);
+	}
 out_locked:
 	if ((mask & MAY_WRITE) && test_bit(IMA_DIGSIG, &iint->atomic_flags) &&
 	     !(iint->flags & IMA_NEW_FILE))
@@ -438,7 +450,7 @@ int ima_file_mprotect(struct vm_area_struct *vma, unsigned long prot)
 	inode = file_inode(vma->vm_file);
 	action = ima_get_action(file_mnt_user_ns(vma->vm_file), inode,
 				current_cred(), secid, MAY_EXEC, MMAP_CHECK,
-				&pcr, &template, NULL);
+				&pcr, &template, NULL, NULL);
 
 	/* Is the mmap'ed file in policy? */
 	if (!(action & (IMA_MEASURE | IMA_APPRAISE_SUBMASK)))
@@ -896,7 +908,7 @@ int process_buffer_measurement(struct user_namespace *mnt_userns,
 		security_task_getsecid_subj(current, &secid);
 		action = ima_get_action(mnt_userns, inode, current_cred(),
 					secid, 0, func, &pcr, &template,
-					func_data);
+					func_data, NULL);
 		if (!(action & IMA_MEASURE) && !digest)
 			return -ENOENT;
 	}

security/integrity/ima/ima_policy.c

@@ -35,6 +35,7 @@
 #define IMA_FSNAME	0x0200
 #define IMA_KEYRINGS	0x0400
 #define IMA_LABEL	0x0800
+#define IMA_VALIDATE_ALGOS	0x1000
 
 #define UNKNOWN		0
 #define MEASURE		0x0001	/* same as IMA_MEASURE */
@@ -79,6 +80,7 @@ struct ima_rule_entry {
 	bool (*uid_op)(kuid_t, kuid_t);    /* Handlers for operators       */
 	bool (*fowner_op)(kuid_t, kuid_t); /* uid_eq(), uid_gt(), uid_lt() */
 	int pcr;
+	unsigned int allowed_algos; /* bitfield of allowed hash algorithms */
 	struct {
 		void *rule;	/* LSM file metadata specific */
 		char *args_p;	/* audit value */
@@ -90,6 +92,14 @@ struct ima_rule_entry {
 	struct ima_template_desc *template;
 };
 
+/*
+ * sanity check in case the kernels gains more hash algorithms that can
+ * fit in an unsigned int
+ */
+static_assert(
+	8 * sizeof(unsigned int) >= HASH_ALGO__LAST,
+	"The bitfield allowed_algos in ima_rule_entry is too small to contain all the supported hash algorithms, consider using a bigger type");
+
 /*
  * Without LSM specific knowledge, the default policy can only be
  * written in terms of .action, .func, .mask, .fsmagic, .uid, and .fowner
@@ -646,6 +656,7 @@ static int get_subaction(struct ima_rule_entry *rule, enum ima_hooks func)
  * @pcr: set the pcr to extend
  * @template_desc: the template that should be used for this rule
  * @func_data: func specific data, may be NULL
+ * @allowed_algos: allowlist of hash algorithms for the IMA xattr
  *
  * Measure decision based on func/mask/fsmagic and LSM(subj/obj/type)
  * conditions.
@@ -658,7 +669,7 @@ int ima_match_policy(struct user_namespace *mnt_userns, struct inode *inode,
 		     const struct cred *cred, u32 secid, enum ima_hooks func,
 		     int mask, int flags, int *pcr,
 		     struct ima_template_desc **template_desc,
-		     const char *func_data)
+		     const char *func_data, unsigned int *allowed_algos)
 {
 	struct ima_rule_entry *entry;
 	int action = 0, actmask = flags | (flags << 1);
@@ -684,8 +695,11 @@ int ima_match_policy(struct user_namespace *mnt_userns, struct inode *inode,
 			action &= ~IMA_HASH;
 			if (ima_fail_unverifiable_sigs)
 				action |= IMA_FAIL_UNVERIFIABLE_SIGS;
-		}
 
+			if (allowed_algos &&
+			    entry->flags & IMA_VALIDATE_ALGOS)
+				*allowed_algos = entry->allowed_algos;
+		}
 
 		if (entry->action & IMA_DO_MASK)
 			actmask &= ~(entry->action | entry->action << 1);